1. FIELD OF THE INVENTION
The present invention relates to a volume control system for a compressor unit of the type wherein it is not permissible that the ratio of delivery pressure relative to intake pressure during a partly loaded operation, i.e., pressure ratio, is considerably differentiated from the pressure ratio during normal loaded operation. An example of this type of compressor is a screw compressor of the dry type.
2. DESCRIPTION OF THE PRIOR ART
The dry type screw compressor comprises a pair of screw rotors disposed in a housing in non-contact relationship with each other. No oil is fed into the housing during operation. In this type of compressor, if the compressor is controlled such that the intake flow is throttled while the compressor is subjected to the same delivery pressure, as is in an oil-injection type screw compressor, the intake pressure becomes close to a partial vacuum with a result that the pressure ratio is extraordinarily increased. In addition, the dry type screw compressors are not sufficiently cooled to avoid undue rise of the delivery temperature, which would cause accidental contact or sticking between screw rotors themselves or between the rotors and the housing due to the thermal expansion of the rotors. Thus, a volume control method has been employed for the dry type screw compressor, wherein the intake flow is throttled and simultaneously the delivery port is open to the atmosphere so as to eliminate the increase of the pressure ratio.
However, if the intake flow is completely stopped, the pressure ratio will become unduly high. Thus, another control has been proposed wherein the intake flow is not completely stopped but a small amount of gas is admitted into a compressor to restrain the increase of pressure ratio as much as possible. Such a control method and a system used to carry out the method are disclosed in U.S. Pat. No. 3,367,562. The control system disclosed in the U.S. Pat. has problems which will be discussed hereunder.
A hydraulic actuator comprising a housing (27) and a diaphragm (26) is employed to open a throttle valve (6) and close a pressure relief valve (22) and to keep these valves in these positions.
In loaded operating condition of the compressor unit, the pressure relief valve (22) is subjected to the delivery pressure of the high-pressure compressor (3) through a conduit (17). The force produced by this delivery pressure is of a considerable magnitude. This force could be reduced by reducing the diameter of the pressure relief valve (22). However, if the valve diameter were reduced, the resistance to the flow of the gas through a valve seat of a correspondingly reduced diameter will be correspondingly increased, so that the power to drive the compressors (2) and (3) cannot be reduced even during unloaded operation.
The diaphragm (26) divides the interior of the housing (27) into two chambers (28) and (29). The maximum pressure in the second chamber (29) will be equal to the delivery pressure of the high-pressure compressor (3) (with the compressor unit disclosed in U.S. Pat. No. 3,367,562, the pressure in the second chamber is equal to the delivery pressure of the low-pressure compressor (2). Considering the fact that leakage through the pressure relief valve (22) when in its closed position, i.e., during loaded operation of the compressor unit, must be avoided, the inner diameter of the housing (27) of the hydraulic valve actuator must be at least as large as from 2 to 3 times of the diameter of the valve (22). Thus, a large-sized valve actuator is required.
The use of a fluid pressure to operate the throttle valve (6) and the pressure relief valve (22) disadvantageously involves a substantial time delay from the moment when a pressure-sensitive switch actuator (77) associated with a gas reservoir or tank (14) emits a signal to the moment when the throttle valve (6) is open and the pressure relief valve (22) is closed or vice versa. Accordingly, the pressure in the tank (14) will be considerably varied from a level below a predetermined lower limit set by the switch actuator (77) to a level above a predetermined upper limit set by the actuator. The compressor unit control system disclosed in the U.S. Patent, therefore, fails to provide a good response and thus causes an unduly high power consumption.
The throttle valve (6) and the pressure relief valve (22) are linked by a rod (24). Thus, the pressure relief valve (22) is required to be disposed at a place near to the throttle valve. Consequently, the length of the pressure relief pipe (17) is increased with a resultant increase in the resistance of the pipe to the flow of the pressurized gas discharged therethrough during an unloaded operation of the compressor unit. The increase in the length of the pressure relief pipe (17) also has direct and indirect problems that the choice of the places at which a silencer and cooler (19), an intercooler (9), an after cooler (13), the air tank (14), etc. of the compressor unit is considerably limited.